There is a need for thin crystalline ribbons and films of many materials such as silicon and other semiconductors. These ribbons are often very costly and difficult to produce. For example, thin wafers of monocrystalline semiconductor materials are generally produced from monocrystalline boules grown by the Czochralski technique. The preparation of the thin wafers from large crystal boules requires slicing and polishing, is a costly and time consuming technique, and includes an inherent waste of much of the boule. Consequently, much effort has been directed toward growing thin monocrystalline ribbons that need only be scribed and broken to be used.
As reported, for example, by Chalmers, LaBelle, and Mlavsky in "Edge-Defined Film-Fed Growth," Journal of Crystal Growth, vol. 13/14, pp. 84-87, 1972, thin ribbons of semiconductors and insulators have been grown through elongated openings in a melt cover and/or with guide posts. In these latter techniques the crystal is pulled in a direction perpendicular to the surface of the melt using the geometric properties of a slot in the melt cover or guide posts to establish the transverse cross section of the resulting ribbon. However, satisfactory larger ribbon widths of appropriate thickness are produced only slowly and with reduced commercial advantage. This is because the probability of grain-boundary defects is proportional to the square root of the pull velocity, as confirmed by Pfeiffer, et al., in "Pattern Formation Resulting from Faceted Growth in Zone-melted Thin Films," Physical Review Abstracts, vol. 16, No. 10, May 15, 1985.
Ribbons of semiconductors have also been produced by pulling substantially in a horizontal plane from a melt surface, as disclosed by Bleil in U.S. Pat. Nos. 3,681,033 and 3,759,671. This technique produces ribbons much faster than the previous methods. However, pulling a ribbon too quickly or at too great an angle from horizontal introduces grain boundaries and imperfections which degrade the performance of circuitry placed on the semiconductor surface. This fact has been described in detail in U.S. Pat. Nos. by Jewett 4,289,571 and Kudo in 4,329,195. Moreover, the necessary controls to implement the process and produce very thin crystal ribbons and films of good quality are difficult to manage and thus the commercial advantage is reduced.
It has also been proposed to horizontally produce flat ribbons by zone melting techniques. This has been described by Geis et al. in "Materials Research Society Symposium," vol. 13, p. 477, 1983, and Omachi et al., "Ge-Seeded Crystallization on SiO.sub.2 by Using a Slider System with RF Heated Strip Heater," Electronics Letters, vol. 19, No. 8, Apr. 14, 1983. However, such techniques develop freezing isotherms nearly perpendicular to the pulling direction and have not been wholly satisfactory for producing thin ribbons of high quality at low cost. Such difficulties may be avoided or corrected by recrystallization, according to this invention. Other ribbons or films may be similarly zone recrystallized.
In some applications, it is desirable to prepare monocrystalline films or semiconductor or other materials on an insulator substrate such as semiconductor on insulator, or SOI, structures. This can be accomplished by depositing powders, polycrystalline, or amorphous films of materials on an insulator material and then subjecting the film material to a zone recrystallization step. The desired monocrystalline structure can be given to the film material through a variety of "seeding" techniques, including beginning and maintaining the seeding process at a location away from the insulator material, thereby inducing a monocrystalline form over the insulating material or causing the desired crystal structure to be propagated through the ribbon cross section.
For ribbons or SOI structures, a variety of methods for supplying the zone-melting energy have been used, including lasers and graphite heaters with energy-focusing means. Methods for inducing electrical currents in the semiconductor and other materials, by exposing them to a high frequency electric field, have been used to recrystallize the cylindrical boules in a particularly energy efficient manner. However, induction methods have not been applied to ribbons because of the inability to control the shape and size of the zone of recrystallization. It is desirable to use current induction to cause zone recrystallization in a thin layer of semiconductor and other materials in a manner allowing the shape and size of the zone to be controlled.